The present invention relates broadly to a method and apparatus for forming a headpiece on a preformed tubular body, and is more particularly concerned with providing a mold cavity in which to form a headpiece on a thin-walled preformed plastic or laminate tube, such as a bag intended for use in an aerosol can.
Various methods are used for forming a headpiece on a preformed tubular body. In one commonly-used method, a preformed tubular body is placed upon a internal shoulder forming member which has a diameter smaller than the inside diameter of the tubular body which allows a slight tolerance or clearance between the internal shoulder forming member and the tubular body. Then the internal shoulder forming member is inserted into an opposing female forming member such that the top end of the tubular body carried on the internal shoulder forming member extends beyond the head end of the internal shoulder forming member and abuts an inwardly curved inside surface of the opposing female member. The curved inside surface of the female member forms and bends or rolls the end of the tubular body inwardly effecting a tight sealing fit with the outside surface of the tubular body.
In order to prevent thermoplastic material from escaping through the tolerance or clearance between the inside surface of the tubular body and the internal shoulder forming member, or so-called "blow-up" of the molten material during the molding of the headpiece, it is necessary to maintain a tight sealing fit also between the internal shoulder forming member and the inside surface of the tubular body. Up to the present time, this has been accomplished by providing the inside surface of the female forming member, at a location directly opposite the top outer edge of the internal shoulder forming member, with a slightly smaller inside diameter than its overall inside diameter so that simultaneously as the internal shoulder forming member reaches the end of its insertion stroke this portion of the female member with the reduced diameter, together with the top outer edge of the internal shoulder forming member, will "squeeze" the tubular body to form a tight seal.
However, the above approach requires the maintenance of extremely precise tolerances and presents complications when one desires to automatically load preformed tubes onto the internal shoulder forming member, particularly, in the case of loading small tubes with thin-walled constructions, such as contemplated for use as bag elements in aerosol cans.
The present invention overcomes this problem by using a internal shoulder forming member, with a substantially undersized diameter relative to the inside diameter of the tube body, having an expandable washer at its head end. The use of this expandable washer, in conjunction with the undersized diameter of the internal shoulder forming member, enables a loose fitting installation of the tube body automatically onto the internal shoulder forming member prior to the headpiece forming operation.
The conically-shaped washer is normally in a contracted form, being retained by the bottom end of the cylindrical head of a internal neck forming member which is mounted for axial movement within the head end of the internal shoulder forming member. After the internal shoulder forming member with the tube body loosely fitted thereon is inserted into the female forming member and when it has moved therein toward its closed position relative to the female member at the end of its forward stroke, the top end of the cylindrical head of the internal neck forming member makes contact with the female member and depresses or retracts into the internal shoulder forming member as the internal shoulder forming member continues to advance forward to the end of the stroke. As the final advance of the internal shoulder forming member cause the internal neck forming member to retract, the head end of the advancing internal shoulder forming member expands the periphery of the washer to effect a peripheral contact of the washer with the inside diameter of the tube body establishing a tight sealing fit between the internal shoulder forming member, the tube body and the female forming member to define a mold cavity. While the forming elements are maintained in this position, molten thermoplastic material is introduced into the mold cavity to form the headpiece on the preformed tube body.